In pressurized water reactors (PWRs), light water is used as nuclear reactor coolant and neutron moderator, serves as high-temperature and high-pressure water that does not boil in the entire primary system. The high-temperature and high-pressure water is sent to a steam generator by which steam is generated by heat exchange, and the steam is then sent to a turbine generator to produce electricity.
In such pressurized water reactors, the core is cooled by introducing coolant into the nuclear reactor from outside, and circulating the coolant. In other words, the coolant flows in through a plurality of coolant inlet nozzles formed on a reactor vessel, flows down a downcomer portion provided between the reactor vessel and a core barrel, and reaches a lower plenum. The coolant then flows upward by being guided by the inner spherical surface of the lower plenum in the upper direction, passes through a lower core plate and the like, and flows into the core. The coolant flowing into the core absorbs thermal energy generated by fuel assemblies that form the core, thereby cooling the fuel assemblies. The temperature of the coolant becomes high, then the coolant flows upward to an upper plenum, and is discharged through a coolant outlet nozzle formed on the reactor vessel.
In such pressurized water reactors, the lower plenum includes structures such as a radial key that supports the core barrel and an in-core instrumentation guide tube for inserting test equipment into the fuel assemblies. Accordingly, the coolant supplied to the lower plenum through the downcomer portion collides with the structures and is dispersed. Consequently, the flow rate distribution of the coolant in the radial direction and the circumferential direction of the core is not uniform.
Therefore, for example, Patent Document 1 discloses a method of providing a connection plate to straighten the flow of coolant in the lower plenum. In the core structure of a nuclear reactor disclosed in Patent Document 1, the lower plenum includes a connection plate whose outer peripheral shape is asymmetric to the main flowing direction of the coolant. Accordingly, it is possible to prevent the generation and formation of separation vortices. Because the coolant uniformly flows into the core, the pressure drop of the coolant flow can be reduced, thereby stabilizing the coolant flow.
[Patent Document 1] Japanese Patent Application Laid-open No. 2005-009999